Two of the toughest perks in the game to unlock are tough because they revolve around using grenades. These perks are the Sentinel and Vaporize perks. Vaporize requires you to have someone die by making them drop their own grenade a total of five times. The Sentinel perk is for killing five enemies with grenades that you've thrown back at them.

The easiest place to do this is the London Nautica, which is Chapter 6. Just after you enter the compound, there will be a large guard robot at the end of the hall. Take a left and hide in the room. Aim at the stairs. Almost every enemy that goes to this landing will try to throw a grenade. This is the absolutely perfect place to farm the "Vaporize" achievement/trophy.

Sentinel is also very easy once you know a trick behind it. Even grenades that you throw yourself can be picked back up and tossed at the enemies, which makes this a lot easier. At the same spot in the game, instead of turning left and entering the room, just run straight at the robot and the enemies surrounding him.

Toss a grenade on the ground, pick it up, and then throw it in their faces. You should be able to get three with one grenade most of the time. If not, just reload the checkpoint and keep at it.

It is possible to play a level from the original Wolfenstein FPS, Wolfenstein 3D.

In most of the chapters that take place in the resistance headquarters (there are multiple), go to the top floor. Tucked away in the corner is a dirty mattress and a picture of Wolfenstein 3D taped to the wall.

Look down at the mattress. If you look at it long enough, eventually it will prompt you to press a button in order to enter the "Nightmare". The "Nightmare" in question, of course, is the Wolfenstein 3D level mentioned earlier.

There are two timelines in the game. You can choose to save Fergus or Wyatt at the end of the first chapter by picking one of them to be the sacrifice for Deathshead's experiment. There are no real gameplay differences, but who you pick will change dialogue and some cut-scenes experienced in the story.